Thursday, 19 March 2020

Synthesizer Build part-23: DIGISOUND 80.6 LOWPASS FILTER.

A very cool AS3320 design that sounds amazing! With verified stripboard layout and new schematics.

After having taken out the Sequential Pro One lowpass filter to make room for the Korg filter, I needed a new use for the AS3320 chip that was inside it. I found the Digisound 80 point 6 lowpass filter module on this awesome website that has all the schematics for the entire Digisound 80 modular synthesizer.
You can configure the filter for any type you want (it's all in the original text) but we are going to build the lowpass filter because for subtractive synthesis the lowpass is the best sounding and most useful of all the filters in my opinion.
I first made a new schematic drawing because the original had those zigzag lines for resistors and I find the rectangular way of drawing resistors easier and you can put the value of the resistor inside the box. Makes it less complicated to look at imho.
Anyway, here's the new schematic drawing:

So after that was finished I made a stripboard layout. It is verified because I used this twice and both times the filter worked perfectly. Make sure you work accurately though because I wouldn't consider this a beginners project. The layout includes a second audio output with 3 times the gain of the original output. This is of my own design and is not included in the schematic drawing. More about this further down the article:

(Last revised: 19-March-2020. Added a second audio output with 3 times gain compared to the normal output.)

Print only:

Bill of Materials:

The panel potmeters used are all 100K linear types but the value isn't that important. Since they are all connected to either a powersupply voltage or an audio signal you can use any value you like from 10K upwards. I myself used three 100K potmeters for the Coarse, Fine and Resonance and I used three 10K potmeters for the audio and CV level controls. This works just fine. If you're going to build this filter for a monophonic modular synthesizer then build it without the "Fine" control and use just the "Coarse".  Fine is more for Polyphonic systems, it has only a very small influence on the Cut-Off Frequency. I'm telling you this now so you can adapt your panel design. Most filters have just one C.O.Freq control.
This is a 24dB/Octave, 4-pole LPF and it is self oscillating unlike the Prophet One filter I used this chip in earlier. That one refused to self oscillate. I used simple ceramic capacitors for the 220pF caps and this works fine. There's no need for fancy polystyrene caps ^___^.
The 1µF electrolytic cap C7 at the input may seem to have the wrong polarity. Usually a cap like that would have the positive pole connected to the point where the signal comes from and negative to where the signal needs to go. In this case it is mounted correctly because the input opamp is an inverting buffer with a negative gain reducing the amplitude of the 10Vpp input signal by a third to an amplitude the chip can handle. In the output buffer the signal is then inverted again to a positive signal with a gain of 3 to give us the original amplitude.

Calibrating the filter:
There are three trimmer potmeters on this print and you can set them as follows:
RV8, the 100K trimmer, is used to trim away the DC voltage on the audio output. Measure the DC output voltage with nothing connected to the input and turn RV8 until it reads zero.
RV7, the 20K trimmer is an interesting one. It's used to have the filter track 1V/Octave oscillators correctly but I simply tune it for best sound. If you have a squarewave on the input and you turn this trimmer you can clearly hear the over-tones, the harmonics, change in pitch. You should be able to hear the frequency beating effect of the note from the VCO against the tone of the resonance. Trim until there's no frequency beating but also listen to the tone while changing the cut-off frequency and trim until it sounds right to you. There's a full description of the proper way to calibrate this filter in the original text, which is in the link I mentioned earlier in this article.
The last trimmer is the one in series with the current limiting resistor for the AS3320. Simply measure the resistance and set it so the total resistance of the trimmer with the 1K resistor equals 1,5K. You could also just put in a 1K5 resistor but turning this trimmer does have a little influence on the sound but you'll have to try it to know what I mean. I just set it to 1K5 and left it at that. Turning this trimmer all the way to zero resistance won't damage the chip though, eventhough it needs a 1K5 current-limiting resistor, 1K won't hurt it. The one thing I learned building this filter is that the AS3320 is quite a robust chip. I made a few mistakes building it the first time and the chip has had voltages (through resistors) placed on the wrong pins, short circuits and all sorts of other mishaps but it survived all that without a scratch. Thank goodness because I only have one of them at the moment :) Luckily I was able to test if the chip still worked by placing it back inside the old Prophet One filter and seeing if that still worked. That was very useful.
Anyway, you can use this filter with a dual 12 Volt power supply, but in that case the current limiting resistor should be 1,2K in total. But it's really not that important. Simply connect it to +/-12V and it should work fine.

This filter sounds amazing! It has its own distinctive sound and I can not say it sounds like the Korg or the ARP or the Moog Ladder filter. It sounds like a Digisound 80 filter, although it comes veeeeeeery close to the ARP in sound. This one sounds a bit more well behaved, if you know what I mean. The sound of the Resonance is clearer than in the ARP which has a Rensonance that is sharper and rougher in sound. But that's the only  difference I could hear so it occupies a solid second place over the Korg-MS20 and the Moog Ladder filter in my personal top 5. The ARP filter is still number one because it's a real rebel and I love it. But hey, remember, this is all just my personal preference. You may judge it quite differently. Actually, I find myself using this filter more often than the ARP filter somehow.
The output from the Digisound 80 LPF is a bit more attenuated than the other filters I built and that's why I used the left-over opamp in IC-1 as an output buffer with a gain of 3. There's a 150K resistor from pin 6 to ground and double that value, 330K, as feedback resistor from pin 6 to pin 7. (You can use any value over 10K for both resistors, as long as the feedback resistor is twice the value of the resistor to ground.) This brings the volume up to the same level as the other filters I made. As I mentioned earlier, the amplitude is first divided by 3 and then multiplied by 3 again in the output opamp but, at least in my filter, I found the sound still lacking in volume compared to the other filters. That's why I wired up the left-over opamp as an amplifier with an extra gain of 3 to bring it up to normal. I'm not sure if it's just my filter or if this is normal, that's why I left the original output un-touched so you can use it if you think my solution is too loud. You could also install a potmeter of 500K instead of the feedback resistor so you can manually set the gain. (Put a 50K resistor in series with the potmeter so the feedback loop can't go to zero Ohm.)
The original audio output is marked on the layout so you can choose which one you want to use.
The first time I build this filter I had used a coupling capacitor of 4,7µF over the audio output because I measured a big DC offset voltage on the audio output, but then I read the original text and found out you can trim that away with trimmer RV8 so I took out the cap and trimmed the DC away and now it's all as it should be.
This filter has an input for 1V/Octave but unlike the ARP filter it's not necessary to use this. The filter will work fine without it but if you connect a 1V/Oct. source to it, the filter will track the octaves better. The sharp synthesizer sound we all love, will be more prominent if you use the 1V/Oct input. It actually makes the filter sound better.
Like I mentioned before, this filter has 2 potmeters for the Cut-Off Frequency but I advise to only use the 'Coarse' control. Fine is only for Polyphonic synths. I included it in my build so I could hear its effect and write about it here, but I normally don't use it. It stays in the middle position because turning it just changes the sound a tiny bit. It can be handy though to tune it into a certain harmonic frequency because this filter brings out the harmonics of a square wave really well, but all in all; leave it out.

Here's a picture of the finished panel built into the synth.:

As you can see in the picture I also included a bypass switch on my panel so I can put other filters in series with this one and if I only want to use one filter I can bypass this one and send the signal straight to the next one without having to change the patch cables. For instance, having this filter in series with the Korg MS-20 in High Pass mode sounds pretty amazing too! That way you have a Band-pass filter made up of two different filters. The bypass switch is only connected to the 'Audio-1' input though. If you want to see the wiring diagram for this switch, you can find it in the article about the Moog Ladder Filter, in which I also installed a switch like this.

Here's a little video with a demonstration of the sound of the Digisound 80.6 LPF:

So that's the Digisound 80.6 lowpass filter done. I can really recommend building this. It has some very recognizable synthesizer sounds that you should really have available in your synth. Make sure you use good quality stripboard though. The first one I build had problems because strips of copper would become loose and break. So I rebuilt it with better quality stripboard. Make sure you use the filter with the 1V/Oct connected to get the best out of it.

Okay, thanks for being here and if you have any comments or questions just put them in the comments below and they will be answered asap.


  1. Hey! Just built this..worked great first try! Just wondering, I used Tl072's since I had no 82s. Would there be a difference in tone/sound in any way?

    1. That's great No I never noticed any difference between the 72 and 82. You can use both without problems.